The introduction of cyclosporin as an immunosuppressive agent revolutionized the treatment of transplant rejection and also aided in the treatment of certain autoimmune diseases such as juvenile onset diabetes. Remarkably these therapeutic advances are achieved with inhibition of only 50% of the activity of calcineurin, the target of both cyclosporin and FK506, indicating that calcineurin is a particularly critical molecules in the development of the normal immune response. The use of cyclosporin is limited by renal, CNS, and pancreatic toxicity all of which are now thought to be due to a block in the actions of calcineurin. In the present proposal we will construct transgenic animals containing a mutant cyclophilin (Cph/t-) expressed only in lymphocytes that interacts with a modified harmless cyclosporin (CsA/t-). In this mice we can completely block the function of calcineurin in selective cell types. We will then determine if the development of autoimmune diabetes in NOD mice can be completely prevented by suppression of the Ca2+/calcineurin pathway in T cells. Since cyclosporin treatment also appears to result in the development of long-term tolerance to transplanted tissues we will determine if complete suppression of the Ca/2+/calcineurin/NF-AT pathway enhances the development of tolerance. If successful, these experiments will direct efforts to treat auto immunity to the development of specific inhibitors of the calcineurin/NF-AT pathway and lay the groundwork for treatment of graft-versus-host disease by inserting the modified cyclophilin gene into stem cells at the time of transplantation and treating with the modified cyclosporin A.